Science

Bitesize Science - http://www.bbc.co.uk/schools/ks2bitesize/science - Interactive activities for life science, chemistry, and physical science.
EdHeads - http://www.edheads.org - Interactive activities for a variety of science areas.
Fantastic Contraption - http://fantasticcontraption.com/index.php - Online physics puzzle
Glogster EDU - http://edu.glogster.com - Online poster creator - add text, images, SchoolTube videos, links, and graphics. Great to replace those tri-fold or poster presentations!
Invention at Play - http://www.inventionatplay.org/playhouse_tinker.html - Interactive physics activities.
Life Size Blue Whale - http://www.wdcs.co.uk/media/flash/whalebanner/content_pub_en.html - Project this on the screen and move around for student to get an idea of how large the blue whale really is.
sketchfu - http://sketchfu.com/home - Online whiteboard. Draw images for play-back. Create a class account for students to log in and share drawings of story maps, processes, solving problems, and drawings.